Continuation of analysis of the mechanism of specific aminoacylation of tRNA will involve the enzymes from E. coli that activate arginine, lysine and leucine and their cognate tRNA's. It is proposed to complete the analysis of the primary structure of tRNALys and begin studies on the effects of modifications of specific bases. Chemical modifications will also be carried out with tRNAArg and the fragments previously isolated that bind to the enzyme. Purification of large amounts of aminoacyl-tRNA synthetases with the aid of affinity chromatography on Sepharose linked to specific tRNA will be carried out to permit initiation of studies on the chemistry of the purified proteins. Kinetic studies of the arginine enzyme will be continued to attempt to define the intermediate steps in the overall process, which are obscured by the requirement for tRNA as an activator. The equilibria of the partial reactions of the synthetases, in which the enzyme-aminoacyl-AMP complex reacts either with pyrophosphate or tRNA, will be measured.